As it is known by a person skilled in the art, in UMTS FDD system (“Frequency Division Duplex”) (and other similar and equivalent systems) downlink signals intended for user mobile stations are processed before being transmitted in a multiplexed manner. This process consists in independently spreading the different user symbols (associated to different HSDSCH channels) at possibly different rates with their respective orthogonal short (channelization or spreading) codes, then adding the resultant streams at chip level and finally scrambling the sum chip sequence by a cell-specific long (scrambling) code prior to synchronous transmission. For instance, in DS-CDMA systems where multi-rate is considered for providing various levels of Quality of Service (QoS), one uses Orthogonal Variable Spreading Factor (OVSF) codes with factors ranging from 4 to 512 in powers of 2 as orthogonal short spreading codes for spreading the user symbols.
It is recalled that in the UMTS standard four QoS classes are defined with differing delay and ordering needs: conversational (low delay and strict ordering—for instance voice data), streaming (modest delay and strict ordering for instance video data, interactive (modest delay and modest ordering—for instance web browsing data) and background (no delay guarantee and no ordering—for instance bulk data transfer).
HSDPA has been proposed to increase the downlink data throughput by using fast physical layer retransmission and transmission combining and link adaptation controlled by each base station (Node B).
In HSDPA two of the main features of W-CDMA (wireless-CDMA) are disabled: the variable spreading factor and the fast power control. They are replaced by adaptive coding rate, adaptive modulation and extensive multi-code operations. The spreading factor is fixed to 16, and a user mobile station can use up to 15 (channelization or spreading) codes simultaneously, which enables a large dynamic range of HSDPA link adaptation and maintains a good spectral efficiency. The scheduling process is done in the Node Bs so that they had the possibility to allocate all the capacity to one user, if necessary and if the channel conditions make this strategy efficient.
HSDPA offers four main properties: allocation of multiple access codes for HSDPA service, fast scheduling of allocated codes, link adaptation and hybrid automatic repeat request (HARQ). To support these properties, two additional types of channels have been introduced. In the downlink, one or more high-speed shared control channels (HS-SCCHs) broadcast HSDPA channel assigned identities, transport format and HARQ process identifier. In the uplink, the high-speed dedicated physical control channel (HS-DPCCH) carries the status reports for HARQ and the channel quality indicators (CQIs).
In CDMA downlink transmission, although the transmitted user signals at the base station side are orthogonal, they are no more orthogonal at the mobile station front-end due to the multipath effect of the propagation channel between the transmitter and the receiver. This loss of orthogonality consequently causes inter-code interference (also known as multi-user interference (MUI) or multi-access interference (MAI)), inter-chip interference and inter-symbol interference in the symbol estimates performed by the communication receivers of the user mobile stations. Communication receivers that are within an optimal or close-to-optimal category, i.e. multi-user detectors (MUDs) and interference cancellers (ICs), most of the time require the knowledge about the signal and the channel parameters of all active users so as to mitigate the multipath effect and to detect the desired data stream in the most reliable way.
However the possibility to implement MUDs or ICs in mobile stations is limited due to their high complexity and due to the fact that the transmission parameters of all the users are usually unknown. A very practical and highly utilised suboptimal solution consists in using a conventional Rake Receiver that performs a matched filter operation on the code of the desired user, in which the multi-user interference is considered as an additional white noise. However, when a small number of spreading codes is used to achieve high data rates (as it is the case in HSDPA), the performance of the Rake receiver decreases due to the fact that the multipath interference becomes significant and the correlation characteristics of the spreading sequences are destroyed.
For this reason it has been proposed to use an equaliser in a HSDPA communication receiver in order to restore the orthogonality between the user codes and to limit the interference, and therefore achieving very high data rates.
It is recalled that an equaliser is a linear filter used for equalizing a channel, which is common for the signals (symbols) of all the users located in a same cell, with a delay of a chosen number of chips, in order to maximise the signal to interference plus noise ratio (SINR). This linear filter is generally of the LMMSE type. It comprises first and second stages. The first stage is provided for determining the filter weights of the linear filter, while the second stage is provided for data filtering by means of the filter weights.
Several types of equaliser for HSDPA have been proposed. They provide remarkable gains in performance. In particular they propose the usage of adaptive algorithms which approximate the chip level linear minimum mean-squared error (LMMSE) receiver expression derived in the following two articles:    M. Lenardi, D. T. M. Slock “SINR maximizing equaliser receiver for DS-CDMA”, EUSIPCO 2000, 10th European association for signal processing conference, 4-9 Sep. 2000—Tampere—Finland,    M. Lenardi, D. T. M. Slock “A RAKE structured SINR maximizing mobile receiver for the WCDMA downlink”, Asilomar 2001, 35th IEEE Annual Asilomar Conference on Signals, Systems and Computers, 4-7 Nov. 2001—Pacific Grove, USA.
In general the adaptive equalisers are either symbol level solutions updated at the primary common pilot channel (PCPICH) symbol rate or chip level solutions updated at chip rate (i.e. the filter adaptation operation at chip rate and chip level).
Pilot-aided equaliser design for CDMA systems is not as trivial as it is for TDMA systems like GSM. In TDMA systems the common pilot signal is time-multiplexed with the payload data. Therefore it is not interfered by any user data but only by additive white Gaussian noise (AWGN). In CDMA systems such as the UMTS FDD downlink, the pilot data (PCPICH) is code-multiplexed with all the other existing users and control channels. Therefore, since PCPICH chip power is only 10% of the base station transmitted chip power, a high level of interference impacts it and it cannot be used efficiently for training equaliser weights at chip rate.
Instead, in the article:
of C. D. Frank, E. Visotsky and U. Madhow “Adaptive Interference Suppression for the Downlink of a Direct Sequence CDMA System with Long Spreading Sequences”, Journal of VLSI Signal Processing, vol. 30, no. 1, pp. 273-291, March 2002, one despreads the received signal with the pilot channelization code, and then suppresses most of the interference over the PCPICH signal.
In this method, in order to determine the N filter weights, the first stage comprises a tap delay line connected to N branches each comprising a descrambler and a despreader (for despreading the received signals with a PCPICH channelization code Cch,256,0 (i.e. the pilot signal)), a linear filter comprising N input filter taps respectively connected to the N branches and a regression input and arranged to output a PCPICH symbol estimate, and an adaptation module for subtracting the PCPICH symbol estimate from a desired symbol (i.e. the correct (and known) PCPICH symbol) to output an error signal feeding the regression input. Although this is a quite effective method against interference, due to the long despreading operation, adaptation is considered only once every 256 chips, which correspond to one PCPICH period. This drives the technique to be slow in tracking the time-varying channels.
It is recalled that in HSDPA the downlink received signals correspond generally to multiplexed HSDSCH channels (“High-Speed Downlink Shared Channels”) associated with different 16 chips long channelization codes. However none of the state of the art equalisers has been designed in order to exploit the characteristics, properties and features of HSDPA. So, they are not really optimised for HSDPA.